We propose to investigate two new hypotheses based on recent work by ourselves and others which pertain to cellular mechanisms of canalicular bile formation and their relationship to regulation of intracellular pH, intracellular volume, Na,K-ATPase cation pumping and sodium-coupled solute transport. First, we postulate that active proton transport across the hepatocyte plasma membrane mediates hepatic bicarbonate secretion and much of bile acid-independent bile formation and is involved as well in regulation of hepatic intracellular pH. Second, we postulate that Na,K-ATPase cation pumping is regulated by the rate of transport of sodium-coupled solutes such as bile acids via changes in intracellular sodium concentration so as to maintain the electrochemical sodium gradient, and that certain hormones and drugs have a primary effect on Na,K-ATPase cation pumping which in turn affects the electrochemical sodium gradient, transport of sodium-coupled solutes and bile formation. These hypotheses will be investigated by studies of transport and/or electrophysiology in the intact perfused liver, cultured rat hepatocytes and liver plasma membrane vesicles using techniques developed by the investigators. The findings are expected to provide important new insight into cellular mechanisms of canalicular bile formation and into the pathophysiology of intrahepatic cholestasis. They will also clarify the nature of the dynamic relationship between Na,K-ATPase cation pumping and sodium-coupled solute transport. Such fundamental information is necessary for a complete understanding of important processes other than bile formation which are dependent on sodium-coupled transport and are common to all animal cells or are characteristic of specialized tissues. These processes include regulation of intracellular pH and volume, gluconeogenesis, muscle relaxation, and transepithelial transport of water, sodium, bicarbonate, chloride and glucose. Information derived from these studies is also expected to aid in understanding the pathophysiology of human diseases other than cholestasis which are associated with abnormal active and passive ion transport including cystic fibrosis, obesity, cancer, and hypertension.